1. Field of the Invention
The present invention relates to the controlling method for transferring and separation in an electrophotographic copying apparatus with an electrostatic separation system.
2. Description of the Prior Art
FIG. 1 shows a part of the rough constitution for the primary part of a copying apparatus wherein an image holder is formed in a drum shape, as an example of copying apparatus of this type. In this diagram, 1 is a photoreceptor drum having a photoreceptor layer of selenium or the like on the surface thereof and 2 is a charging electrode that causes the photoreceptor of the photoreceptor drum 1 to be charged by the corona discharge. The numeral 3 is an optical system that guides the reflected light corresponding to the original image on the platen (unillustrated) to the surface of the photoreceptor drum 1 and thus forms an electrostatic latent image, and 4 is a developing section that applies toner to the electrostatic latent image and thus forms a toner visible image. The numeral 5 is a first paper feeding roller that feeds out transfer paper loaded in the paper feeding cassette 6 to the prescribed position, and 7 is a second paper feeding roller that feeds out transfer paper so that the toner visible image on the surface of the photoreceptor drum 1 and the transfer paper are moved in synchronization. The numeral 8 is a transferring electrode that transfers the toner visible image on the drum surface to the transfer paper by means of corona discharge, 9 is a separation electrode that separates the transfer paper from the surface of the photoreceptor drum 1 by means of corona discharge and 10 is a transporting section that transports the transfer paper separated from the surface of the photoreceptor drum 1 to the fixing section 11. The numeral 12 is a cleaning section that cleans the toner remaining on the surface of the photoreceptor drum 1 and 13 is a preliminary exposure section that illuminates the entire surface of the photoreceptor drum 1 prior to the start of the copying cycle.
As one of the controlling methods around the photoreceptor drum 1 of the copying apparatus, there has been known a method wherein the photoreceptor drum 1 is rotated [FIG. 2(a)] concurrently with depressing the "ON" copy-button and at the same time, the actions of charging, copying and cleaning are started [FIG. 2(b)]. The aforesaid actions are stopped simultaneously after the completion of image formation. Copying in this case includes the processes of exposure, developing, transferring and separation. This controlling method has the merit that the control is simple, but it has the drawback that the discharging time of the electrode is long, contamination of the wire takes place in a short period of time, and the lamp to neutralize an electric charge in the area other than the image forming area should be provided against the photoreceptor drum 1 between the charging electrode 2 and the developing section 4 because charging is made continuously. As a controlling method to eliminate these drawbacks, there has been known a method wherein, as a timing chart for the interval of two copies in FIG. 3 shows, the photoreceptor drum 1 is rotated [FIG. 3(a)] and cleaning as well as preliminary exposure are made concurrently with the "ON" of the copy-button and later, charging is made [FIG. 3(b)] and further, the transferring and separation are made [FIG. 3(c)] after a certain period of time.
However, the aforesaid conventional controlling method has a common drawback that the first copy time cannot be shortened because the control of the neutralizing for transferring and separation is improper. This point is described as follows based on the results of experiments. First of all, if the leading edge of the image is at point P.sub.1 that is just before the preliminary exposure section 13 in the former conventional example, a band-shaped low density area appears at the central portion of the image on the particular first copy (under the condition that a Se-Te drum with a diameter of 120 mm.phi. is used as a photoreceptor drum 1 and the line speed thereof is 150 mm/sec. and A3 is applied as a transfer paper size). Further, the measurement of the surface potential on the photoreceptor drum 1 at the point of the developing section 4 with the use of an original of solid black gives the characteristic curve shown in FIG. 4. The point A that is sharply dropped on the characteristic curve corresponds to the existence of the aforesaid low density area. The reason why the low density area appears is that the surface of the photoreceptor drum 1 that faces the separation electrode 9 when the copying is started is charged negatively by the AC discharge in the photoreceptor of the present example, and these negative charges are not removed by the preliminary exposure section 13 and are carried over to the next step. (What the preliminary exposure section 13 can do is only to bring the surface charged positively on the photoreceptor drum 1 close to OV.) Therefore, in this conventional example, the leading edge position needs to be selected of the point of P.sub.2 which is positioned before the transferring electrode 8 and by doing so, the first copy time is made about 7.5 sec. The condition is quite the same even for the latter conventional example and if the leading edge position of the image is set at P.sub.1, the first copy has an uneven density and even the second copy has a white band. Therefore, in the same manner as the former conventional example, the first copy time needs to be long.